Wavefront sensor using a surface acoustic wave diffraction grating

Abstract

A wavefront sensor for detecting the slope of an input wavefront. A surface acoustic wave reflective diffraction grating is positioned at a focal point of the wavefront, and generates surface acoustic waves at two primary frequencies f 1 and f 2 . The reflective diffraction grating produces a first AC shearing interferogram between two like diffraction orders generated by the f 1 and f 2 surface acoustic waves. A photodetector array is positioned to detect the shearing interferogram at a two dimensional array of zones, and the phase of the output signal for each zone is representative of the local slope of the wavefront in the direction of shearing, thus producing the slope in one direction. Complete two dimensional wavefront slope information is obtained by shearing the wavefront in a second orthogonal direction in substantially the same manner utilizing a second surface acoustic wave diffraction grating and a second photodetector array.

What is claimed is: 
     
       1. An arrangement for detecting the slope of an input wavefront, comprising: a. means for focusing an input wavefront to a focal point;   b. a surface acoustic wave reflective diffraction grating positioned substantially at said focal point and generating acoustic surface waves at two primary frequencies f 1  and f 2  to produce a first AC shearing interferogram, sheared in a first direction, between two like diffraction orders generated by the f 1  and f 2  surface acoustic waves;   c. photodetector means positioned to detect the first AC shearing interferogram at a two dimensional array of zones and for producing an output signal for each zone, such that the phase of each output signal is representative of the local slope of the wavefront in said first direction in that zone;   d. a beam splitter means in the input wavefront for directing a first portion thereof to said surface acoustic wave reflective diffraction grating, and for directing a second portion of the input wavefront to a second surface acoustic wave reflective diffraction grating positioned substantially at the focal point of said second portion of the input wavefront, said second surface acoustic wave reflective diffraction grating generating acoustic surface waves at two primary frequencies f 1  and f 2  to produce a second AC shearing interferogram, sheared in a second direction substantially orthogonal to said first direction, between two like diffraction orders generated by the f 1  and f 2  surface acoustic waves; and   e. a second photodetector means positioned to detect the second AC shearing interferogram at a two dimensional array of zones and for producing an output signal for each zone, such that the phase of each output signal is representative of the local slope of the wavefront in said second direction in that zone.   
     
     
       2. An arrangement for detecting the slope of an input wavefront as claimed in claim 1, wherein each surface acoustic wave reflective diffraction grating is formed on a piezoelectric base having a first smooth surface, positioned substantially at said focal point, on which a pair of interleaved finger electrodes are deposited, and including means for generating and applying across said pair of electrodes electrical signals at said primary frequencies f 1  and f 2  to generate surface acoustic waves at frequencies f 1  and f 2  on said smooth surface. 
     
     
       3. An arrangement for detecting the slope of an input wavefront as claimed in claim 2, each first smooth surface including a reflective coating thereon to increase its reflectance of the input wavefront. 
     
     
       4. An arrangement for detecting the slope of an input wavefront as claimed in claim 2, including at least one acoustic energy sink on each piezoelectric base for absorbing acoustic energy incident thereon. 
     
     
       5. An arrangement for detecting the slope of an input wavefront as claimed in claim 2, each piezoelectric base having a second smooth surface positioned optically behind the first smooth surface, each second smooth surface being transversely inclined at an angle relative to the first smooth surface to cause light to be reflected away from each two dimensional detector means. 
     
     
       6. An arrangement for detecting the slope of an input wavefront as claimed in claim 5, including a mounting base for each piezoelectric base, each mounting base including a recess therein below each piezoelectric base in the direction in which the input wavefront is being focused to a focal point, and a beam dumping aperture in each mounting base for radiation in the focused wavefront which passes through both the first and second smooth surfaces. 
     
     
       7. An arrangement for detecting the slope of an input wavefront as claimed in claim 2, including a second pair of interleaved finger electrodes deposited on each first smooth surface for receiving and detecting the generated surface acoustic waves, whereby the power in the generated surface acoustic wave can be monitored. 
     
     
       8. An arrangement for detecting the slope of an input wavefront as claimed in claim 1 or 2, each two dimensional photodetector means being positioned to detect a shearing interferogram formed between two first diffraction orders.